Meso–Neoproterozoic coated grains and palaeoecology of associated microfossils: The Deoban Limestone, Lesser Himalaya, India

Organically preserved microfossil assemblages occurring in association with the coated grains (ooids and pisoids) are being reported from the petrographic thin sections of the black chert. Samples have been collected from the Deoban Limestone, Garhwal Lesser Himalaya. Like any other Proterozoic microfossil assemblage, present assemblage also exhibits domination of cyanobacterial community and rare occurrence of bacterial and acritarchean affinity.The microfossils occur in three principal palaeoecological modes:a)Clast bound microfossils: These are found in detrital grains which also served as a nuclei for ooid growth.b)Epilithic or interstitial microfossils: These occur on the outer surface of the coated grains as well as within the cement.c)Euendolithic microfossils: These microorganisms actively bored into the coated grains.Coated grains comprising the microbial structures can be considered as the indicators of shallow water environments. Their size is at least partly dependant on current strength. At larger sizes, increasing rates of abrasion may come to equal rates of carbonate precipitation or accretion. Since filamentous population of specific dimensions dominate the present assemblage, it is inferred that tubular filaments ranging in diameter from 4 to 5 μm played the most significant role in the genesis of coated grains irrespective of their mode of occurrence as an epilith, endolith or as the clast bound microfossils. The assemblage adds one more record of Proterozoic endoliths, which are rarely recorded world-wide.